设点为(X,Y)
则由该点到点(1,2)的距离和到点（3,4）的距离之差等于2根号2
得：根号[(x-1)^2+(x-2)^2]-根号[(x-3)^2+(x-4)^2]=2*根号2
根号[(x-1)^2+(x-2)^2]=根号[(x-3)^2+(x-4)^2]+2*根号2两边平方
[(x-1)^2+(x-2)^2]=[(x-3)^2+(x-4)^2]+4*根号2*根号[(x-3)^2+(x-4)^2]+8
8x-28=4*根号2*根号[(x-3)^2+(x-4)^2]
2x-7=根号2*根号[(x-3)^2+(x-4)^2]
4x^2-28x+49=4x^2-48x+50
x=1/20